Brexit talks haven't yet made sufficient progress to begin negotiating a new relationship between Britain and the European Union, European Council president Donald Tusk said Tuesday, but he's now "cautiously optimistic" about the way ahead.

After meeting with British Prime Minister Theresa May, Tusk said he believed Britain had finally abandoned an unrealistic "having a cake and eating it" approach to Brexit.

Britain triggered a two-year countdown to Brexit in March, but since then talks have made little progress. Negotiators are meeting in Brussels this week for a fourth round of discussions, as EU officials warn Britain that time is running out.

Britain wants to start discussing future trade and security relations with the bloc, but EU officials say that can't happen until there has been "sufficient progress" on the terms of the divorce.

In a bid to break the logjam, May made a conciliatory speech in Florence, Italy, last week, saying the U.K. would be willing to pay into EU coffers during a two-year transition phase after it formally leaves the bloc in March 2019.

'Realistic tone'

Tusk said Tuesday that he felt Britain had finally abandoned an unrealistic belief that it could have its cake and eat it — leave the EU while keeping all the advantages of membership.

"I feel cautiously optimistic about the constructive and more realistic tone in the prime minister's speech in Florence and of our discussion today," Tusk said after meeting May at 10 Downing Street in London.

"There is no sufficient progress yet, but we will work on it," he added.

Other EU officials also gave a cautious welcome to May's speech, but say more detail is needed. Chief negotiator Michel Barnier said Monday he was "keen and eager" to understand how May's words would be turned into a negotiating strategy.

The EU says it won't discuss future relationships until it has guarantees on Britain's outstanding financial commitments, the rights of more than four million EU and British citizens affected by Brexit and the status of the Irish border.

Johnson tries to resassure

EU leaders are scheduled to assess at an Oct. 19-20 meeting whether Britain has made "sufficient progress" on the divorce arrangements for negotiations to move on. It's likely they will decide it's too soon and will reassess the decision until a meeting in December.

In another sign of Britain's more conciliatory tone, Foreign Secretary Boris Johnson tried to reassure eastern Europeans that they will remain welcome after Brexit.

When Britain leaves the bloc, it will end the automatic right of EU nationals to live and work in the U.K., and that has left many worried for their futures.

Johnson told Romania's Agerpres news agency that some 500,000 Romanians currently in Britain are "hugely valued members of our society."

"We would be crazy to let them go back to Romania, we want to ensure … their rights are protected," said Johnson, who is on a visit to the Czech Republic, Romania and Slovakia.